1. Field of the Invention
This invention relates to a shaped article of an oriented tetrafluoroethylene polymer. More particularly, the present invention is concerned with a shaped article comprising an at least biaxially oriented tetrafluoroethylene polymer of ultrahigh molecular weight (hereinafter frequently referred to as ultrahigh molecular tetrafluoroethylene polymer) and having specific values with respect to a draw ratio, a specific gravity and an orientation release stress. The shaped article has excellent creep resistant and gas barrier properties, and accordingly can be advantageously used as a sealant, a lining material, a bearing pad or a sliding pad.
2. Discussion of Related Art
Polytetrafluoroethylene is now an important engineering material because it has various excellent properties. In many industries, polytetrafluoroethylene is now widely used as a sealant material, such as a gasket, a packing, a lining material or the like, as the polymer is excellent in its resistance to chemicals, heat, low temperatures, and has low frictional properties, non-staining properties, tack-free properties, electrical insulating properties and the like.
The melt viscosity of polytetrafluoroethylene is extremely high as compared with those of ordinary thermoplastic resins such as polystyrene, polyethylene, polyamide, acrylic resin and the like. Polytetrafluoroethylene has a melt viscosity as high as about 10.sup.11 poise even at a temperature of about 380.degree. C. higher than its melting temperature (327.degree. C.), whereas the melt viscosities of the ordinary thermoplastic resins are only about 10.sup.3 to 10.sup.4 poise at their melting temperatures. Due to such an extremely high melt viscosity, polytetrafluoroethylene cannot be molded by the customary molding techniques such as melt extrusion, injection molding and the like which are generally employed for the molding of these ordinary thermoplastic resins. The methods which are useful for molding polytetrafluoroethylene are limited to special methods, for example, those called compression molding, ram extrusion molding and paste extrusion molding, in which a polytetrafluoroethylene powder is compressed at an appropriate pressure to obtain a molded article and the molded article is sintered at a temperature which is higher than the melting temperature thereof, followed by cooling. For these methods, reference may be made, for example, to pages 53 to 54 of Satokawa et al "Fusso Jushi (Fluoro Resins)" published by the Nikkan Kogyo Shimbun Ltd., Tokyo, Japan, in April, 1976.
Further, it is noted that the temperature up to which polytetrafluoroethylene can be used under no-load conditions is 260.degree. C. However, in many applications, polytetrafluoroethylene is used under load, and the critical temperature up to which an article of polytetrafluoroethylene can be stably used is far lower than 260.degree. C., depending on the structure, configuration and field of application of the article. Illustratively stated, the polymer is formed into various shaped articles, such as gaskets, sliding pads and the like, and the articles are often used under a heavy load. Polytetrafluoroethylene has an inherent property of creeping under a load, especially at high temperatures. For example, when a polytetrafluoroethylene article is used as a gasket, the gasket tends to be compressed and laterally flattens, due to the inherent creeping property of the polymer under high temperature and pressure, which undesirably leads to leakage at the fitting intended to be sealed by the use of the gasket. To prevent such leakage, it is periodically needed to retighten the bolts of abutting flanges so that a fluid tight seal is ensured with the gasket. However, such periodic retightening is a time consuming process, especially where flanges are numerous and awkwardly located. Similar problems occur with respect to a packing, a sliding pad, a bearing pad and the like as well. To obviate such problems, the article of polytetrafluoroethylene must be used at a temperature which is far lower than the above-mentioned 260.degree. C. However, articles such as packings, gaskets, sliding pads and bearing pads must often be used at high temperatures. Therefore, there is a strong demand in the art for polytetrafluoroethylene articles having improved heat resistance.
In the field of gaskets, primarily, two measures have been taken to deal with the problem of polytetrafluoroethylene creeping. One is to incorporate a filler material such as glass fiber, graphite, carbon fiber and zirconium oxide into the polytetrafluoroethylene. The other is to combine polytetrafluoroethylene or a filler material-incorporated polytetrafluoroethylene as mentioned above with a material having less of a tendency to creep, as seen in jacketed and spiral wound gaskets. The effect of such measures is apparent from the following data. The critical temperatures up to which the polytetra- fluoroethylene-based gaskets can be stably used are 100.degree.-120.degree. C. in the case of a flat gasket consisting purely of polytetrafluoroethylene, 150.degree.-200.degree. C. in the case of a flat gasket comprised of polytetrafluoroethylene and a filler material, 150.degree.-180.degree. C. in the case of a jacketed gasket and 260.degree.-300.degree. C. in the case of a spiral wound gasket. As seen from the foregoing, with respect to the flat gasket, the incorporation into polytetrafluoroethylene of a filler material, tends to improve the above-mentioned critical temperature by about 50.degree. to 100.degree. C. However, the flat gasket comprised of polytetrafluoroethylene and a filler material is not satisfactory since the gasket is still insufficient in creep resistant properties at high temperatures, and since some types of filler materials cause the gasket to have a decreased resistance to chemicals. On the other hand, the jacketed and spiral wound gaskets tend to have problems with respect to the resistance to chemicals, and they are disadvantageous in that for these, a high clamping pressure is needed and thus they cannot be formed into complex forms. Therefore, there is still a strong demand in the art for polytetrafluoroethylene-based gaskets which have excellent creep resistant properties and are free from the above-mentioned disadvantages. With respect to the sealants, other than gaskets, such as valve sealants, e.g. ball valve seats and gate valve seats, and dynamic sealants, e.g. gland packings, U packings and V packings, polytetrafluoroethylene is also advantageously used when they are required to have anticorrosive properties. Improvement of the compression creep resistance of the article is also strongly desired in the art for the reasons as mentioned above with respect to gaskets.
Polytetrafluoroethylene is also important as an anticorrosive lining material. However, it has a drawback in that it tends to be accompanied by a blistering phenomenon, especially in the pipes for a gas-phase fluid, which is fatal from the viewpoint of the purpose of the lining. Hence, improvement of the blistering resistant properties of the polytetrafluoroethylene article is strongly desired in the art.
With a view to improving the creep resistant properties of polytetrafluoroethylene articles, proposals have been made. (See U.S. Pat. No. 4,388,259 and U.S.S.R. Patent No. 405,732). Specifically, to improve the creep resistant properties of a polytetrafluoroethylene gasket, it has been proposed, as disclosed in U.S. Pat. No. 4,388,259, to manufacture a gasket by a method comprising (a) heating a fluorocarbon polymer sheet of predetermined thickness to within the temperature range at which the fluorocarbon polymer enters the gel state; (b) compressing said heated sheet to a thickness less than the above-mentioned predetermined thickness; (c) cooling the sheet in its compressed state; and (d) cutting the sheet, in its compressed, cooled state, into a gasket. However, the gaskets as manufactured according to the method of the patent do not have desirable creep resistant properties. On the other hand, with respect to U.S.S.R. Patent No. 405,732, in one of the two Examples described in the specification thereof there is disclosed the rolling of polytetrafluoroethylene to attain a high orientation of the polymer. Rolling is generally employed to produce a monoaxially oriented continuous sheet. In the roll orientation, it is necessary to conduct rolling while applying the tensile force in the lengthwise direction of the sheet and to release the tensile force only after cooling the sheet. Rolling is not suitable for effecting biaxial or multi-axial orientation of the polymer molecules. The other Example of the U.S.S.R. patent discloses compression of a preformed sheet of polytetrafluoroethylene in a press mold corresponding in form and dimension to the ultimate packing so as to cause the ratio of the thickness of the preformed sheet to the thickness of the ultimate packing to be 1.2 or more, which ratio may be regarded as a draw ratio. However, according to this method, the maximum value of the ratio is at most about 2.0 and a shaped article of a highly oriented polytetrafluoroethylene cannot be obtained. Therefore, the products obtained in the U.S.S.R. patent are not desirable with respect to the orientation of the tetrafluoroethylene polymer molecules and the creep resistant properties.